1. Field of the Invention
The present invention relates to a scanning signal processing system, and more particularly to an analog scanning signal processing system for converting an analog scanning signal having a substantially constant shading profile to a multi-level signal.
2. Description of the Prior Art
When an analog signal is converted to a multi-level of multi-state signal such as bi-level or tri-level signal, the analog signal voltage is compared with a reference voltage or reference voltages to produce the multi-level signal. The conversion precision depends on the precisions of the analog voltage and the reference voltage.
In a facsimile or an optical character reader (OCR), optical information such as characters on a document sheet is scanned by photo-electric converter means to produce an analog video signal, which is then converted to a multi-level video signal. The analog video signal may be produced by superimposing the information signal on a background representing a plain paper. When the optical information representing the darkness or brightness on the document sheet is converted to a bi-level video signal, the conversion precision depends on a distribution of illumination on the document sheet and image forming conditions of an optical system including lenses. When a line sensor is used, the conversion precision further depends on a distribution of sensitivity of sensor elements of the line sensor. Thus, while the document sheet is plane (e.g uniformly white), the scanned analog video signal (referred to as a white scan signal train) may not have a constant voltage level but have a specific profile. This profile is reproduced in each scan with a substantially same pattern. Such a characteristic distribution of the analog video scan signal representing background (state of no information) is called shading. A general or relative characteristic distribution is called a shading profile, and a characteristic distribution including voltage level is called a shading characteristic, in this specification. When the analog information signal containing the shading is compared with a constant reference voltage to convert it to a multi-level signal, a correct multi-level information signal is not produced.
Optical compensation and electric compensation for the shading have been proposed. In the optical compensation, the illumination of the document sheet may be adjusted by rearranging a light source or disposing a filter or a mask so that a white scan signal train having a constant signal level is produced. This method needs a complex and large device and takes long time for adjustment and is hard to attain a sufficient effect.
The electrical compensation includes a way of controlling a gain of a video amplifier to produce a shading-free video signal and another way of effecting compensation in a signal processing step of converting an analog signal containing shading to a multi-level signal.
It has been proposed to store the shading in a memory and read it out in a scan cycle to compensate the shaded signal.
Japanese Patent Application No. 52-9761 a shading signal read out of a memory.
In a facsimile, the analog video signal is usually converted to a bi-level (digital) signal. In order to correctly convert a black information signal superimposed on a white scan analog video signal containing shading to a bi-level signal, a slice signal which varies in accordance with the shading may be used.
Japanese Patent Application No. 48-130508 (Laid-Open No. 50-81419) teaches to compensate a slice signal for discriminating a video signal by a shading signal read out of a read-only memory (ROM).
Co-pending U.S. Patent Application Ser. No. 151,982 by one of the inventors of the present application teaches to carry out shading compensation using a stored shading profile and a peak voltage of the scan signal held in a peak-hold circuit.
It has been desired to carry out the shading compensation with a high precision by a simple circuit configuration without requiring a high speed operation.